Keeping the correct time of day has traditionally been a problem for video receivers in televisions and in video cassette recorders (VCR). The user has been the only source of knowledge for programming the current time. However, typical consumers do not know how, or do not wish, to set or correct the internal clock. As a result, the display on many receivers is never changed from the power-up default display of flashing "12:00 A.M.".
With the incorporation of Extended Data Services (EDS) information into television signals in the United States, current time information will be available in the television signal. EDS information is a data component of the signal that may be included in the vertical blanking interval of an NTSC standard television signal. In particular, EDS data may be included in line 21 of field 2 in an encoded form like that of closed caption data. EDS and captioning will share the bandwidth of line 21 field 2 in a time multiplexed manner. EDS will offer a wide range of useful information such as: program title, network name, and current time of day.
The format of EDS data is specified in the EIA-608 standard for line 21 data services for NTSC television signals. The EDS data component of a television signal is organized in packets of data bytes. Each packet conveys one piece of information, e.g., the current time of day. Each line 21 of field 2 provides two bytes of EDS data. Each byte includes 7 bits of data (bits 0 through 6) and a parity bit (bit 7). Particular data byte values occurring in the first byte of any line 21 of field 2 indicate the start of a packet. Each "start" data byte also defines a "class" of information that is included in the packet. For example, a "current" class packet includes information pertaining to the current program that is being viewed. A "miscellaneous" class packet includes various types of information such as time of day information. As an example, a value of 01.sub.h (i.e., 01.sub.hex) in the first data byte in line 21 of field 2 indicates the start of a "current" class packet while a value of 07.sub.h indicates the start of a "miscellaneous" class packet.
The data byte following a "start" byte, i.e. the second byte in the particular occurrence of line 21, determines the "type" of information in the packet. For example, if the data byte following a "miscellaneous" packet start byte has the value 01.sub.h, the packet "type" is a "time of day" packet. A time-of-day packet includes six bytes (in addition to the start, type, and end bytes) formatted as shown in Table 1. The six bytes define the current time of day relative to GMT. The valid ranges for the various fields are shown in Table 2. In Table 1, the "D" bit in byte 2 determines if daylight saving time is currently being observed, the "L" bit in byte 3 determines if the current year is a leap year, and the "Z" bit in byte 4 determines if the current time in seconds should be set to zero. In Table 2, a "day" value of 1 in byte 5 indicates Sunday and a value of 7 indicates Saturday. The "year" value in byte 6 is selected such that adding the value to 1990 produces the current year.
TABLE 1 ______________________________________ Byte Data b6 b5 b4 b3 b2 b1 b0 ______________________________________ 1 minutes 1 m5 m4 m3 m2 m1 m0 2 hours 1 D h4 h3 h2 h1 h0 3 date 1 L d4 d3 d2 d1 d0 4 month 1 Z -- m3 m2 m1 m0 5 day (of week) 1 -- -- -- d2 d1 d0 6 year 1 y5 y4 y3 y2 y1 y0 ______________________________________
TABLE 2 ______________________________________ Byte Data Bits Valid Range ______________________________________ 1 minute m5-m0 0-59 2 hour h4-h0 0-23 3 date d4-d0 0-31 4 month m3-m0 0-12 5 day (of week) d2-d0 1-7 6 year y5-y0 0-63 ______________________________________
Because time of day is defined in terms of GMT, information defining the time zone of the user is necessary to determine the correct local time. The time zone of the source of a video signal is provided in another type of packet in the miscellaneous class of EDS packets, namely the "local time zone & DST" packet type. The time zone packet includes two bytes. In the first byte, bits b4 through b0 specify a delay in hours (range of 0-23) from GMT for the location of the source of the video signal. Bit b5 of the first byte determines whether the locality of the signal source observes daylight saving time. If bit b5 is a logic 0, i.e. daylight saving time not observed, the "D" bit in the time of day packet is ignored.
The local time zone packet is only inserted in the video signal if reception of the signal is limited to an area within a single time zone. For areas on time zone boundaries, the required time zone information must be entered into the receiver by a user when the receiver is powered up, e.g., as a step executed during a menu of initialization procedures.
As shown in Table 1, the current time of day in EDS data is specified in hours and minutes, but not seconds. A single bit "Z" is provided to indicate if the seconds value is zero at the time of the EDS transmission. If the current time of day is transmitted exactly at the top of a minute, i.e. zero seconds, the "seconds" bit could be set indicating that the receiver's internal seconds clock should be reset to zero. However, providing time information using a "seconds" bit has at least two drawbacks. First, a time of day packet may be transmitted only occasionally (e.g., once every ten minutes) and randomly. A long time interval might be required before the time of day information was transmitted with a zero seconds value. Second, the "zero seconds" bit might not be set, even though it is exactly the top of a minute. In this case the receiver would never have the exact correct time. Correct time, including the seconds component, may be important when controlling features such as starting to record a television program in a video cassette recorder (VCR). Inaccuracies in the seconds component may result in recording beginning early (a portion of an unwanted program is recorded) or late (the beginning of a desired program is not recorded).